1. Field of the Invention
The present invention relates to a hydraulic power booster adapted to produce a boosted output in response to an input, for example, a boosted braking pressure used in a hydraulically operated brake system.
2. Discussion of the Prior Art
A hydraulic power booster as indicated above is known. An example of the power booster used for a hydraulically operated brake is disclosed in laid-open Publication No. 62-155167 of unexamined Japanese Patent Application. This hydraulic power booster includes (a) a reaction piston linked with a brake pedal, (b) a power piston which is axially movable relative to the reaction piston, (c) a valve spool for controlling the pressure in a power chamber, and (d) a lever device disposed in operative association with the reaction piston, power piston and valve spool, for moving the valve spool in response to a relative movement between the reaction and power pistons. For instance, the lever device includes a first link which is pivotally supported at one end thereof by the housing of the booster and which engages the reaction piston at the other end, and a second link which engages the power piston and the end face of the valve spool at the opposite ends, respectively, and which is pivotally connected to an intermediate portion of the first link. Upon depression of the brake pedal, a resulting advancing movement of the reaction piston relative to the power piston causes the first and second links of the lever device to cooperate with each other to advance the valve spool. The movement of the valve spool causes a relatively high pressure to be supplied from a hydraulic power source into the power chamber, whereby the high pressure in the power chamber acts on the power piston, which operates to raise the pressure in a master cylinder operatively connected to the booster. When the master cylinder is used for a vehicle brake system, the pressure produced by the master cylinder with the aid of the booster is applied to a brake cylinder provided for a wheel of the vehicle. In some cases, the boosted pressure in the power chamber is directly applied to the wheel brake cylinder.
In the event the lever device fails to normally function for some reason or other, the valve spool cannot be moved, and the high pressure is not applied to the power chamber, namely, the hydraulic power booster fails to normally operate to boost the input force.